This cavern was about a hundred feet wide and a hundred and fifty in
height. A large mass had been rent asunder by a subterranean
disturbance. Yielding to some vast power from below it had broken
asunder, leaving this great hollow into which human beings were now
penetrating for the first time.

The whole history of the carboniferous period was written upon these
gloomy walls, and a geologist might with ease trace all its diverse
phases. The beds of coal were separated by strata of sandstone or
compact clays, and appeared crushed under the weight of overlying
strata.

At the age of the world which preceded the secondary period, the
earth was clothed with immense vegetable forms, the product of the
double influence of tropical heat and constant moisture; a vapoury
atmosphere surrounded the earth, still veiling the direct rays of the
sun.

Thence arises the conclusion that the high temperature then existing
was due to some other source than the heat of the sun. Perhaps even
the orb of day may not have been ready yet to play the splendid part
he now acts. There were no 'climates' as yet, and a torrid heat,
equal from pole to equator, was spread over the whole surface of the
globe. Whence this heat? Was it from the interior of the earth?

Notwithstanding the theories of Professor Liedenbrock, a violent heat
did at that time brood within the body of the spheroid. Its action
was felt to the very last coats of the terrestrial crust; the plants,
unacquainted with the beneficent influences of the sun, yielded
neither flowers nor scent. But their roots drew vigorous life from
the burning soil of the early days of this planet.

There were but few trees. Herbaceous plants alone existed. There were
tall grasses, ferns, lycopods, besides sigillaria, asterophyllites,
now scarce plants, but then the species might be counted by thousands.

The coal measures owe their origin to this period of profuse
vegetation. The yet elastic and yielding crust of the earth obeyed
the fluid forces beneath. Thence innumerable fissures and
depressions. The plants, sunk underneath the waters, formed by
degrees into vast accumulated masses.

Then came the chemical action of nature; in the depths of the seas
the vegetable accumulations first became peat; then, acted upon by
generated gases and the heat of fermentation, they underwent a
process of complete mineralization.

Thus were formed those immense coalfields, which nevertheless, are
not inexhaustible, and which three centuries at the present
accelerated rate of consumption will exhaust unless the industrial
world will devise a remedy.

These reflections came into my mind whilst I was contemplating the
mineral wealth stored up in this portion of the globe. These no
doubt, I thought, will never be discovered; the working of such deep
mines would involve too large an outlay, and where would be the use
as long as coal is yet spread far and wide near the surface? Such as
my eyes behold these virgin stores, such they will be when this world
comes to an end.

But still we marched on, and I alone was forgetting the length of the
way by losing myself in the midst of geological contemplations. The
temperature remained what it had been during our passage through the
lava and schists. Only my sense of smell was forcibly affected by an
odour of protocarburet of hydrogen. I immediately recognised in this
gallery the presence of a considerable quantity of the dangerous gas
called by miners firedamp, the explosion of which has often
occasioned such dreadful catastrophes.

Happily, our light was from Ruhmkorff's ingenious apparatus. If
unfortunately we had explored this gallery with torches, a terrible
explosion would have put an end to travelling and travellers at one
stroke.

This excursion through the coal mine lasted till night. My uncle
scarcely could restrain his impatience at the horizontal road. The
darkness, always deep twenty yards before us, prevented us from
estimating the length of the gallery; and I was beginning to think it
must be endless, when suddenly at six o'clock a wall very
unexpectedly stood before us.